CH. A'] MICROTOMES AND SECTION KNIVES 287 



preparation with needles and with sharp knives held in the hand. They dealt 

 also with fresh tissue more largely than we do at the present day, and learned 

 also to distinguish tissues by their structure rather than by their artificial 

 coloration. 



It was not, however, on account of the lack of elaborate mechanical de- 

 vices for sectioning and complicated staining methods of the present day, but 

 because they put intelligence and y.eal into their work that made them so suc- 

 cessful. Only those who were "called" made for themselves a laboratory and 

 saw with their brain. Now many are "sent," but few who use the central 

 organ of sight. 



If the reader is interested in the mechanical means for sectioning he is 

 referred to Dr. C. S. Minot's papers on the history of the microtome in the 

 Journal of Applied Microscopy, Vol. VI. In a word, it is now possible with 

 the almost perfect automatic microtomes to make thousands of perfect sections 

 where in 1860 only occasionally could the most expert get tens with his hand 

 sectioning. 



\ 433. Types of Microtomes. There are two great types : (i) The early 

 type in which the preparation to be sectioned is held mechanically and moved 

 up by a screw, the section knife being held in the hand and moved across the 

 object usually with a drawing motion as in whittling (Fig. 228). 



(2) The mechanical type in which both specimen and knife are mechani- 

 cally held and guided, and the operator simply supplies power to the machine. 



In the highest types of the second class automatic microtomes the 

 operator only needs to put the knife and specimen in position and supply the 

 power and sections of any thickness and any number may be produced in a 

 short time. A skilled and experienced person can get better results here as 

 well as with free-hand sectioning or the hand microtome. Even automatic 

 machines work better for skilled workmen. 



As is seen by the accompanying cuts, sometimes the knife is fixed in posi- 

 tion and the object to be sectioned moves, while in other forms the object to 

 be sectioned remains fixed and the knife moves. Furthermore for sectioning 

 paraffin, the knife meets the object like a plane (straight cut), while for col- 

 lodion sectioning the knife is set obliquely and there results an oblique or 

 drawing cut as in whittling. 



\ 434. Section Knives. A section knife should have the following char- 

 acters, (i) The steel should be good. (2) The blade should be slightly hol- 

 low ground on both sides. Why some makers persist in grinding one side 

 flat is a mystery. (3) The edge of the knife should be straight, not curved as 

 in a shaving razor. (4) The back should be parallel with the edge. (5) The 

 blade should be long, 12 to 15 centimenters, as it takes no more time or skill 

 to sharpen a large than a small knife. (6) The blade should be heavy. There 

 was formerly a fashion of making very thin bladed section knives, but that is 

 a great mistake, for the thin blade bends and vibrates in cutting firm tissue 

 and large pieces. There is no possible advantage in a thin bladed section 

 knife for microtome work, but much disadvantage from the lack of rigidity. 



